The present invention pertains to high efficiency switching power supplies, particularly to providing an auxiliary voltage on a switching power supply.
Switching power supplies are widely used in electronic devices. They convert unregulated power from batteries or other sources to a regulated voltage required to operate the device. Especially with hand held and portable devices, high efficiency is required to achieve long battery life. These switching supplies provide a primary voltage, usually 3.3 or 5 volts D.C. to operate digital logic in the device. It is also common to require one or more auxiliary voltages in the operation of the device, such as for modems, serial ports, or for the contrast control voltage for a liquid crystal display.
It is known in the industry to derive an auxiliary voltage from a switching supply by replacing the inductor in the switching power supply with a transformer; the primary winding of the transformer replaces the inductor in the switching power supply. The secondary winding of the transformer is fed to a conventional half wave rectifier and filter capacitor to derive the auxiliary voltage. Unfortunately this scheme while low in cost exhibits poor regulation and low efficiency on the auxiliary voltage, especially over varying load current. Past solutions to the problem of poor regulation on the auxiliary voltage include adding a series regulator, or adding a fixed load on the auxiliary voltage, a type of shunt regulation. Both these solutions decrease the efficiency of the power supply, decrease battery life, and add to component count and therefore cost of the overall device.
What is needed is a method of providing an auxiliary voltage from a switching power supply that is efficient and provides tight regulation of the auxiliary voltage.